The LS Siren Crewe, part of Icebreaker’s underwear collection, is made from the company’s lightest material—150 g/m2. The shirt is primarily made of merino wool (96%), but it also contains some elastane (4%). Icebreaker claims that this material fits well, is naturally odor-resistant, and is breathable.

After wearing the LS Siren Crewe biking, hiking, skiing, and as a casual layer over the past four months, I can say that I’m very impressed.

Fit / Sizing

The Siren Crewe is unbelievably comfortable, not only as a base layer but also as a “normal” shirt. I’ve worn other polyester and nylon long underwear tops from companies such as Under Armor, Nike, Patagonia, and Duofold, and none were nearly as comfortable as the Siren Crewe.

The Siren Crewe is slim fitting without being too tight or restricting. It’s snug, but not skin tight. The material is extremely lightweight and a bit stretchy, allowing the shirt to sit comfortably on my body. The material is soft and not itchy.

I usually wear small shirts, and the Siren Crewe fits comparably to other smalls I’ve used. The sleeves and waist both fit me well— for reference, my torso is 19 inches long while my arms are 23 inches long. The Siren Crewe is long enough that I can tuck it into my ski pants easily, and the sleeves don’t ride up when I’m hunched over a bike or when I’m skiing.

I think I could have gotten away with an extra small (my chest measures 33 inches, my waist measures 26 inches, and my hips measure 33 inches), but personally, I’m happy with the small because I prefer the slightly looser fit.

Function

I need my base layers to be comfortable, warm, and breathable. As I already mentioned, the Siren Crewe is the most comfortable base layer I’ve worn. But how does it stack up in terms of warmth?

I tend to be cold, and so rely heavily on layering and thick jackets to keep me warm. Considering how thin and lightweight the Siren is, I’ve been surprised by how warm it is. I wore it almost every week this past fall instead of a “normal” sweatshirt, and found it to be plenty warm by itself.

I have also worn the Siren Crewe as a base layer while skiing in a range of temperatures this winter. On days around zero degrees F, I wore the crewe under my Arc’teryx Atom Hoody and Dakine Kaitlin jacket. On days where temperatures hovered around freezing, I wore it just under the Dakine Kaitlin jacket or under the Arc’teryx LT Atom Hoody and the Arc’teryx LT Cerium Hoody (review to come). The only base layer I’ve worn that’s warmer than the Siren Crewe is my polyester Duofold long underwear top. But it’s not breathable and gets smelly very fast…

That gets me to my final point. The Siren Crewe is very lightweight and breathable. When I do start to sweat in this top, either when I’m hiking or biking uphill or sidestepping at Alta, it wicks the moisture from my body and the shirt stays dry. In similar situations when I’m wearing my nylon or polyester shirts, I feel stuffy and the shirt gets damp.

Smell / Cleaning

Merino wool is also very odor resistant. It’s remarkable how the Siren Crewe does not smell bad after skiing hard in it for several days in a row, or hiking all day in it. My other synthetic / blend base layers get really smelly, and need to be washed after one use.

Icebreaker recommends that their products be air dried, which isn’t an issue since it dries quickly. And I’ve found that it’s still soft after each wash cycle.

Bottom line

The Siren Crewe is made of an incredible material—merino wool. It’s thin, lightweight, breathable, and comfortable, and it’s also very warm. I’d recommend the Siren Crewe to anyone looking for a base layer for any outdoor activity.

It’s a great option for layering on cold days or while doing strenuous activities. I’ve also found that it can be worn casually around town since it looks so good.